The present invention relates generally to a Coriolis flow meter, and more particularly to a Coriolis mass flow meter of a type using two parallel arched flow tubes.
A mass flow meter (Coriolis mass flow meter) operating on the principle that when both ends of conduits carrying a flowing fluid being measured are supported and the conduits are caused to oscillate around the supporting fulcrums thereof in a direction perpendicular to the fluid""s direction of flow, the Coriolis force exerted onto the conduits (the conduits to which oscillation is applied is hereinafter referred to as the flow tubes) is proportional to the mass flow rate of the fluid is well known. The flow tubes used in the Coriolis mass flow meter are divided into two types; the curved ones and the straight ones.
FIG. 5 is a schematic diagram of a conventional Coriolis mass flow meter of a dual parallel curved tube type. As shown in the figure, flow tubes 1 and 2 comprise two parallel curved tubes (U-shaped tubes), and are driven at the middle part thereof by a drive unit 15 comprising a coil and a magnet so as to resonate with each other at mutually opposite phases. A pair of oscillation sensors 16 and 17 each comprising a coil and a magnet are disposed at locations symmetrical with respect to the mounting position of the drive unit 15 to sense a phase difference proportional to a Coriolis force.
A fluid being measured enters from an external conduit connected to the meter via an entry-side flange 18 into a tubular body 37 and is deflected 90 degrees by an end plate 38, branching equally into two flow tubes 1 and 2. The two fluid flows converge into one at the exit side of the flow tubes 1 and 2, and are deflected 90 degrees by an end plate 36 to be discharged into an external conduit connected to the meter via an exit-side flange 19. It is well known that the natural frequencies of the two flow tubes 1 and 2 can be made always substantially equal even for different types of fluids or for fluids at different temperatures by causing the fluid being measured to flow equally in the two flow tubes 1 and 2. It is also well known that a Coriolis flow meter capable of being driven efficiently and stably and being free from the effects of external oscillations and temperature changes can be provided based on the above construction.
The Coriolis mass flow meter of a curved tube type, which uses and measures the oscillation of the proximal parts of the flow tubes extending in the lateral direction from the tubular body 37, has to secure a length necessary for the proximal parts of the flow tubes extending in the lateral direction. This could inevitably lead to an increase in the size of the meter.
The Coriolis mass flow meter of a straight tube type, on the other hand, has straight flow tubes disposed in the direction of the external conduit, and the straight tubes supported at both ends thereof are oscillated at the middle part thereof in a direction perpendicular to the axis of the straight tubes to detect mass flow rate as a signal of the displacement difference, or phase difference caused by a Coriolis force between the supported parts and middle part of the straight tubes. The Coriolis mass flow meter of a straight tube type can be made into a simple, compact and sturdy construction.
The flow tubes of the straight tube type Coriolis mass flow meter, however, tend to be subjected to temperature fluctuations because the straight tubes must be fixedly supported at both ends thereof. That is, as the temperature of the fluid being measured changes, the flow tubes change its temperature by immediately responding to the change in fluid temperature, while there is a delay in the temperature change of a fixing structural member, such as a chassis to which the flow tubes are fitted. As a result, a difference in elongation is produced between the flow tubes and the fixing structural member, resulting in stresses in the longitudinal direction. This causes the natural frequency of the tube to change due to the changes in spring constant resulting from the stresses. To cope with this, the straight tube type Coriolis mass flow meter must have stress absorbing means, such-as a diaphragm, bellows, etc.
The problem of the longitudinal elongation due to temperature changes can be solved by constructing the flow tube into an arch shape. FIG. 6 is a conceptual diagram of assistance in explaining the operation of the conventional Coriolis mass flow meter having arched flow tubes.
The flow tubes of an arch shape have excellent shock resistance since they can disperse stresses. In the conventional arched tube construction, however, the manifold and the flow tubes are connected in the axial direction of the tube. As illustrated by R in the middle, and two r""s on both sides thereof in FIG. 6(A), the flow tube involves more than three bending steps, making this design unfavorable particularly for a dual-tube construction requiring symmetry. As shown in FIG. 6(B) illustrating the two states of the vertically oscillating flow tubes, the nodes of oscillation that would have been fixed by a base plate are also subjected to oscillation, making accurate measurement difficult.
The present invention is intended to overcome these problems, and it is an object of the invention to provide a Coriolis mass flow meter of a dual arched tube type that are immune to external oscillations, installation conditions, stresses in piping, and thermal stresses by forming the flow tubes into an arched parallel tube type that is favorable in stress dispersion and shock resistance.
It is another object of the present invention to increase rigidity in the direction of leaking oscillation, thereby reducing oscillation leaks without increasing the mass of the proximal parts of oscillation.
The Coriolis mass flow meter according to the present invention comprises two parallel flow tubes 1 and 2, an entry-side manifold 25 for branching the fluid being measured into the two flow tubes 1 and 2 from the fluid inlet, an exit-side manifold 25 for converging the fluid being measured flowing in the two flow tubes 1 and 2 to discharge through the fluid outlet, a drive unit 15 for driving the two flow tubes to cause them to resonate with each other in mutually opposite phases, and a pair of oscillation sensors 16 and 17 installed at symmetrical locations with respect to the mounting position of the drive unit 15 for sensing a phase difference proportional to a Coriolis force. The two flow tubes 1 and 2 are formed into an arch shape, or an arc shape curving in only one direction. The entry- and exit-side manifolds 25 are smoothly bent from the inflow direction of the entry-side manifold and the outflow direction of the exit-side manifold to the joints with the two flow tubes, at which the manifolds rise at a predetermined angle so as to be connected to the flow tubes 1 and 2 in alignment with them. This construction enables the flow tubes of an arched parallel tube type having excellent stress dispersion and shock resistance, resulting in a mass flow meter that is less affected by external oscillations, installation conditions, stresses in piping, and thermal effects.
The Coriolis mass flow meter according to the present invention comprises a sealed pressure-resistant case 31 of a substantially cylindrical shape in the axial direction, with the openings of the cylindrical part thereof at both ends closed by end plates connected with a smooth contour, and entry-side and exit-side manifolds 25 connected at the corners of the cylindrical part thereof in such a manner that the manifolds 25 pass through the corners. This construction increases rigidity in the direction of leaking oscillation, thereby reducing oscillation leaks without increasing the mass of the proximal parts of oscillation.
The Coriolis mass flow meter according to the present invention has a pair of integrally formed disc-shaped flanges at the entry-side and exit-side manifolds, to which both ends of the pressure-resistant case are fixedly fitted, with the cross-sectional shape of the pressure-resistant case made into an oval shape having a major axis oriented in the curved direction of the flow tubes, the length of the major axis smoothly reduced from the axial center toward both ends thereof into a substantially circular shape near both ends over a predetermined length.
The Coriolis masse flow meter according to the present invention has temperature sensors on the pressure-resistant case and near the joints connecting the flow tubes and the manifolds to compensate for the thermal effects of the distance between the fixed ends on both sides of the flow tubes and the thermal effects of the rigidity of the flow tubes.